Federal bankruptcy court Judge Francis Bason Jr. yesterday approved the appointment of an investment adviser for United Press International, resolving a problem that has impeded efforts to sell the news service. UPI's creditors, management, stockholders and union had agreed earlier that a single entity was needed to solicit and evaluate offers for the company but had disagreed on who should take the job and how much to pay for such services.

Under the compromise approved yesterday, two investment houses will serve jointly as investment adviser. Bear Stearns & Co., the choice of UPI management, will work with Ladenburg Thalmann & Co., which was selected by Media News Corp., UPI's sole stockholder. The two lowered their fee by altering a formula for calculating their commission, based on the final sale price.

Although numerous media companies and individual investors have expressed interest in buying all or part of UPI, only one firm offer exists on the table now -- and it expires Monday. A group of unidentified investors offered to buy UPI July 1, pledging to pay off about $17 million of its debts and provide working capital. But UPI's creditors have not acted on the offer and will not meet before the deadline. The investors' attorney, David Rubenstein, said yesterday that the offer will not be extended beyond the deadline.